The partial oxidation of hydrocarbons, for example methane or natural gas, in the presence of a catalyst is an attractive process for the preparation of mixtures of carbon monoxide and hydrogen, known in the art as synthesis gas. The partial oxidation of a hydrocarbon is a highly exothermic reaction and, in the case in which methane is the hydrocarbon, proceeds by the reaction: EQU 2CH.sub.4 +O.sub.2 .fwdarw.2CO+4H.sub.2
A number of process regimes have been proposed in the art for carrying out the catalytic partial oxidation reactions. One regime that is most suitable for application on a commercial scale is to contact the feed gases with the catalyst retained in a fixed arrangement. The literature contains a number of documents disclosing details of experiments conducted into the catalytic partial oxidation of hydrocarbons, in particular methane, employing a wide range of catalysts in a fixed bed arrangement.
A number of academic experiments have been reported in the literature in which catalysts have been employed in the form of fixed beds of catalyst particles.
Thus, A. T Ashcroft et al. ("Selective oxidation of methane to synthesis gas using transition metal catalysts", Nature, Vol. 344, No. 6264, pages 319 to 321, 22nd March, 1990) disclose the partial oxidation of methane to synthesis gas in the presence of a range of ruthenium-containing catalysts. The objective of the experiments was to establish that the partial oxidation process could be carried out under mild conditions and at low temperatures. To this end, the experiments were conducted with a low gas hourly space velocity of 40,000/hr, a pressure of 1 atmosphere and a temperature of about 775.degree. C. The catalyst employed comprised small amounts of a solid, powdered catalyst.
P. D. F. Vernon et al. ("Partial Oxidation of methane to Synthesis Gas", Catalysis Letters 6 (1990) 181-186) disclose a range of experiments in which catalysts comprising nickel, ruthenium, rhodium, palladium, iridium or platinum, either supported on alumina or present in mixed oxide precursors, were applied. Again, the experiments reported are limited to a catalytic partial oxidation process employing only mild operating conditions and using small amounts of catalyst in the form of pellets retained in a fixed bed. The authors report the same experiments in "Partial Oxidation of Methane to Synthesis Gas, and Carbon Dioxide as an Oxidizing Agent for Methane Conversion", Catalysis Today, 13 (1992) 417-426.
R. H. Jones et al. ("Catalytic Conversion of Methane to Synthesis Gas over Europium Iridate, Eu2Ir2O7", Catalysis Letters 8 (1991) 169-174) report the selective partial oxidation of methane using the europium iridium pyrochlore Eu2Ir2O7. The reaction was studied under the mild conditions of a pressure of 1 atmosphere and a temperature of 873K (600.degree. C.). The catalyst was prepared by grinding and subsequent pressing to form pellets. The pelletized catalyst was packed into a porous silica frit and used directly in the experiments.
U.S. Pat. No. 5,149,464 is directed to a method for selectively oxygenating methane to carbon monoxide and hydrogen by bringing the reactant gas mixture at a temperature of about 650.degree. C. to 900.degree. C. into contact with a solid catalyst which is generally described as being either:
a) a catalyst of the formula MxM'yOz, where: PA1 b) an oxide of a d-block transition metal; or PA1 c) a d-block transition metal on a refractory support; or PA1 d) a catalyst formed by heating a) or b) under the conditions of the reaction or under non-oxidizing conditions.
M is at least one element selected from Mg, B, Al, Ln, Ga, Si, Ti, Zr and Hf; Ln is at least one member of lanthanum and the lanthanide series of elements; PA2 M' is a d-block transition metal, and each of the ratios x/y and y/z and (x+y)/z is independently from 0.1 to 8; or
The d-block transition metals are said in U.S. Pat. No. 5,149,464 to be selected from those having atomic number 21 to 29, 40 to 47 and 72 to 79, the metals scandium, titanium, vanadium, chromium, manganese, iron, cobalt, nickel, copper, zirconium, niobium, molybdenum, technetium, ruthenium, rhodium, palladium, silver, hafnium, tantalum, tungsten, rhenium, osmium, iridium, platinum and gold. It is stated in U.S. Pat. No. 5,149,464 that the preferred metals are those in Group VIII of the Periodic Table of the Elements, that is iron, osmium, cobalt, rhenium, iridium, palladium platinum, nickel and ruthenium.
The process described in U.S. Pat. No. 5,149,464 is operated at a temperature in the range of from 650.degree. C. to 900.degree. C., with a range of from 700.degree. C. to 800.degree. C. being preferred. A range of experiments are described in U.S. Pat. No. 5,149,464 in which a variety of catalysts comprising Group VIII metals were tested, including ruthenium oxide, praesidium/ruthenium oxides, pyrochlores, ruthenium on alumina, rhodium on alumina, palladium on alumina, platinum on alumina, nickel/aluminium oxide, perovskites and nickel oxide.
A similar general disclosure of a catalyst for use in the catalytic partial oxidation process is made in International Patent Application publication No. WO 92/11199. WO 92/11199 specifically discloses experiments in which catalysts comprising iridium, palladium, ruthenium, rhodium, nickel and platinum supported on alumina were applied. All the experiments were conducted under mild process conditions, with typical conditions being a pressure of 1 atmosphere, a temperature of 1050K (777.degree. C.) and a gas hourly space velocity of about 20,000/hr.
The experiments described in both U.S. Pat. No. 5,149,464 and WO 92/11199 employed catalysts in the form of solid powdered particles retained in a fixed bed arrangement by packing in a reaction tube between two plugs of silica wool.
European Patent Application Publication No. 303,438 (EPA 303,438) discloses a process for the catalytic partial oxidation of a hydrocarbonaceous feedstock in which a gaseous mixture of the hydrocarbonaceous feedstock, oxygen or an oxygen-containing gas and, optionally, steam, is introduced into a catalytic partial oxidation zone to contact a catalyst retained therein. The catalyst employed in the process may comprise a wide range of catalytically active components, for example palladium, platinum, rhodium, iridium, osmium, ruthenium, nickel, chromium, cobalt, cerium, lanthanum and mixtures thereof. Further, it is stated in EPA 303,438 that materials not normally considered to be catalytically active may also be employed as catalysts, for example refractory oxides such as cordierite, mullite, mullite aluminium titanate, zirconia spinels and alumina. The catalyst may be of a variety of forms, for example sheets of corrugated metal packed to form elongate channels therethrough or wire mesh. However, preference is given in EPA 303,438 to the use of catalysts in the form of extruded honeycomb monoliths. These monoliths comprise a large number of parallel channels extending through the structure in the direction of flow of the feed and product gasses.
European Patent No. 262,947 (EPB 262,947) discloses a process for generating hydrogen by the partial oxidation of a hydrocarbon in which a mixture of the hydrocarbon and oxygen is injected into a mass of a catalyst. The catalyst disclosed in EPB 262,947 comprises platinum and chromium oxide supported on a refractory solid. The support structures described in EPB 262,947 are monolithic honeycomb supports, of the type used in purifying the exhausts from motor vehicles or from chemical plants, and particulate supports, preferably comprising particles having a maximum dimension of from 1 to 4 mm, for example 1.5 mm.
D. A. Hickman and L. D. Schmidt ("Synthesis Gas Formation by Direct Oxidation of Methane over Pt Monoliths", Journal of Catalysis 138, 267-282, 1992)) have conducted experiments into the partial oxidation of methane in the presence of catalysts comprising either platinum or rhodium. The partial oxidation reactions were conducted at substantially atmospheric pressure and at temperatures in the range of from 600 to 1500K (327.degree. to 1227.degree. C.). The catalysts employed were in the form of metal gauzes, metal-coated foam monoliths and metal coated extruded monoliths. The metal gauze catalysts comprised 1 to 10 layers of gauzes of either 40 mesh (40 wires per inch) or 80 mesh. The foam monoliths were of alpha-alumina and described as having an open cellular, sponge-like structure. The samples employed had a nominal porosity of 30 to 50 pores per inch (ppi). The extruded monoliths were cordierite extruded monoliths, having 400 square cells/in.sup.2 and consisted of straight parallel channels giving laminar flows of gases through the channels under the conditions of gas flowrate studied.
J. K. Hockmuth ("Catalytic Partial Oxidation of Methane over a monolith Supported Catalyst", Applied Catalysis B: Environmental, 1 (1992) 89-100) reports the catalytic partial oxidation of methane using a catalyst comprising a combination of platinum and palladium supported on a cordierite monolith body.
European Patent Application Publication No. 576,096 (EPA 576,096) discloses a process for the catalytic partial oxidation of a hydrocarbon feedstock in which a feed comprising a hydrocarbon feedstock, an oxygen-containing gas and, optionally, steam at an oxygen-to-carbon molar ratio in the range of from 0.45 to 0.75 is contacted with a catalyst in a reaction zone under adiabatic conditions. The catalyst comprises a metal from Group VIII of the Periodic Table of the Elements supported on a carrier and is retained in the reaction zone in a fixed arrangement having a high tortuosity. A wide range of carrier materials and structures are disclosed in EPA 576,096, including particles of carrier material, metal gauzes and ceramic foams. Suitable materials for use as carrier materials are said to include the refractory oxides such as silica, alumina, titania, zirconia and mixtures thereof. Alumina is stated as being an especially preferred carrier material.
European Patent Application Publication No. 548,679 (EPA 548,679) discloses a process for the preparation of carbon monoxide and hydrogen by the catalytic partial oxidation of methane in the presence of a catalyst prepared by depositing, as an active component, rhodium and/or ruthenium on a carrier comprising zirconia or stabilized zirconia. The catalyst is described in EPA 548,679 as being of any suitable form, such as finely divided powder, beads, pellets, plates, membranes or monoliths. The catalytic partial oxidation process is described in EPA 548,679 as being conducted at a temperature of from 350.degree. to 1200.degree. C., preferably from 450.degree. to 900.degree. C. under a pressure of up to 300 kg/cm.sup.2 G, preferably lower than 50 kg/cm.sup.2 G. Typical operating gas space velocities are described in EPA 548,679 as being in the range of from 1,000 to 40,000 h.sup.-1, preferably from 2,000 to 20,000 h.sup.-1. The specific examples of EPA 548,679 describe experiments conducted at atmospheric pressure at temperatures of from 300.degree. to 750.degree. C. and space velocities of 16,000 and 43,000 h.sup.-1. In all the experiments described in EPA 548,679, the catalyst was retained in the form of a fixed bed of particles.
The specification of European Patent Application No. 93203331.9 (as yet unpublished) contains a description of a process for the catalytic partial oxidation of a hydrocarbon feedstock in which the hydrocarbon is mixed with an oxygen-containing gas and contacted with a catalyst. The catalyst is retained in a fixed arrangement having a high tortuosity of at least 1.1 and at least 750 pores per square centimeter. The catalyst preferably comprises a catalytically active metal supported on a carrier. Suitable carrier materials are described as including the refractory oxides, such as silica, alumina, titania, zirconia and mixtures thereof. A catalyst comprising a zirconia ceramic foam as carrier is specifically exemplified.
An attractive catalytic partial oxidation process for application on a commercial scale would operate at elevated pressures, typically in excess of 10 bar, for example at around 30 bar, and at high gas hourly space velocities, typically of the order of 500,000 to 1,000,000 h.sup.-1. Due to the thermodynamic behavior of the partial oxidation reaction, in order to obtain a high yield of carbon monoxide and hydrogen at elevated pressures, it is necessary to operate the reaction at elevated temperatures. Temperatures on the order of 1000.degree. C. or higher are necessary for obtaining the yields demanded of a commercial process.
It has been found that a most suitable fixed arrangement for the catalyst for use in the catalytic partial oxidation of hydrocarbons under conditions which would be commercially attractive is one in which the catalyst is retained in the form of a monolithic structure. Catalysts for use in such a process comprise one or more catalytically active components supported on a refractory oxide carrier, the carrier being in the form of a monolith. As mentioned hereinbefore, the partial oxidation reactions are very exothermic, with typical reaction conditions in excess of 1000.degree. C. being required for successful commercial scale operation. However, it has now been found that major problems can arise in the operation of the partial oxidation process when using a catalyst in the form of a monolith. In particular, it has been found that the refractory monolithic catalyst structures are very susceptible to thermal shock under the conditions of very high temperature prevailing in the catalytic partial oxidation process. Thermal shocks arise when the catalyst is subjected to a rapid change in temperature, giving rise to substantial temperature gradients across the catalyst structure. Thermal shocks may arise during a shut-down of a commercial reactor in the case of an emergency, in which case it will be necessary to rapidly cool the reactor and its contents. Thermal shocks may also be encountered by the catalyst during start-up procedures and during normal process operation when fluctuations in the feed rate and composition occur.
The measures needed to prevent the catalyst being subjected to thermal shocks during the operation of a process on a commercial scale are very expensive. Accordingly, there is a need for a catalytic partial oxidation process which combines a high level of selectivity to carbon monoxide and hydrogen, a high level of catalyst stability and resistance to thermal shocks.
Surprisingly, it has now been found that zirconia-based monolith structures offer a significantly greater resistance to thermal shocks under the operating conditions of the catalytic partial oxidation process than monoliths prepared from other materials.